1. Field of the Invention
The present invention relates to the measurement of the absolute position of an object.
2. Description of the Prior Art
A single-axis incremental position encoder is a device for measuring the relative movement of two objects along one axis. Typically, a scale is attached to one of the objects and a read head to the other, the scale having regularly spaced identical markings on it. The read head contains a light source which illuminates the scale and a sensor or sensors for detecting the scale markings. In the simplest case, the read head provides outputs which allow the markings to be counted to keep track of position. Usually, the read head provides some electronic interpolation such that the effective resolution is higher than would be achieved by direct counting of the markings on the scale. In some cases, the outputs are analogue (often two sinusoidal in quadrature) to allow electronics external to the read head to perform the interpolation. As such, an incremental encoder has no knowledge of the read head""s position along the scale. However, a reference mark may be provided on or alongside the scale for this purpose. A sensor in the read head can detect the reference mark and this defines a datum position.
Dual-axis incremental position encoders also exist. In the simplest case, these include two read heads mounted together at right angles to each other, and a scale with periodicity in two usually orthogonal directions, each read head measuring incremental movement in a respective one of the two directions. In other cases, the two read heads are combined together e.g. as shown in U.S. Pat. No. 5,204,524. Dual axis incremental encoders work in a similar way as their single-axis counterparts, but typically produce two separate sets of outputs corresponding to the two axes of movement.
The read head of a single axis absolute encoder typically runs up and down a scale with data written on it, often as binary bits. By reading these bits, either with one detector as the read head passes over the scale, or simultaneously with several detectors, the read head can determine its absolute position. This position is typically relayed from the read head to a control system by means of a serial interface.
Hybrid incremental-absolute single-axis position encoders also exist e.g. European Patent No. 1010967. As it is possible to make incremental encoders with finer resolution than absolute encoders, many absolute encoders also incorporate an incremental channel. The absolute channel gives absolute position accurate to one period of the incremental channel, and interpolation of the incremental channel gives absolute position within the period of the incremental channel to the desired fine resolution. Combined together, the two systems work side by side to give absolute position to the fine resolution.
A xe2x80x9csingle-axisxe2x80x9d encoder may read linear position with a straight scale, angular position, using a curved scale, or position along any other straight or curved line. A xe2x80x9cdual-axisxe2x80x9d encoder may read position in each of two usually orthogonal directions in a plane, position on the surface of a cylinder, or position on any other curved or flat surface.
There are at least two drawbacks with incremental encoders:
When an incremental encoder is powered up, there is no way of knowing the position of its read head. The read head has to be moved relative to the scale to find the reference position(s). In some applications (e.g. linear motors) it is not possible to move the read head until absolute position is known.
In the event of power failure, or if the read head temporarily loses the ability to read the scale, the read head""s position becomes unknown. In some applications (e.g. printed circuit board pick and place machines), subsequent attempts to find the datum position may cause damage to the work or machine.
In situations where two axes of movement must be measured, one solution to these problems would be to use two single axis absolute encoders. However, in many applications this is not possible. Another drawback is that Abbe errors result from the necessary distance between the read heads and the point at which the measurement is required. In some cases, a more compact solution is required anyway.
According to a first aspect the invention provides a measurement scale comprising readable bits having generally constant pitch extending in at least one direction, for enabling incremental measurement, and at least one sub-set of the bits which define an absolute position in the said at least one direction.
The boundary of the or each sub-set may be defined, by symbols.
The bits may be formed as a matrix of areas extending in two directions, each area defining a bit and the at least one sub-set may be a block of such areas for defining the absolute position in at least one of the said two directions.
Preferably the areas form a generally chequered pattern and the areas of the at least one sub-set are distinguishable from the general pattern by means of a variance in the optical properties of one or more of the areas of the sub-set.
The two directions mentioned may be two orthogonal directions.
According to a second aspect the invention provides a measurement scale comprising readable bits having generally constant pitch extending in two directions, for enabling measurement and at least one sub-set of the bits which define an absolute position in the said two directions.
The invention provides also, according to a third aspect a system for measuring absolute position in two directions comprising a scale according to one of the aspects above and a scale reader for reading the sub-set in order to determine the absolute position.
The scale reader may be an optical reader having an optical detector adapted to read the scale optically. Preferably the detector is an array of photodetectors.
Preferably the reader includes a microlens array for focusing an image of the bits of the scale onto the detector, and the system includes a scale illuminator, adapted to provide a short burst of light to provide a substantially instantaneous image for the reader, during relative movement between the scale and the scale reader. The reader may be adapted to read at least one whole sub-set of bits during the relative movement.
According to a fourth aspect the invention provides a system for measuring absolute position in two directions comprising a scale and a scale reader relatively movable in the two directions with respect to the scale, the scale being provided with a matrix of bits in which different sub-sets of bits define different absolute positions in each of the two directions and the scale reader being adapted to read the bits for deriving, from the read sub-set, the absolute position of the reader on the scale in the two directions.
Preferably the matrix of bits enables incremental measurement to take place.
According to a fifth aspect the invention provides a method for measuring absolute position on a scale comprising the steps of:
providing a scale comprising a matrix of bits and sub-sets of the bits formed as words;
providing a scale reader having a scale image detector;
forming an image of at least one whole word at the detector;
capturing the image; and,
decoding the image to obtain an absolute position defined by the word.